As is generally known, the production of solid sodium cyanide from its solutions takes place through the process steps of crystallization, dehydration, drying or according to The principle of spray drying.
All these process steps are known and described in the widest variety of embodiments.
Also known is the further processing to form compacts, briquettes or tablets, as is imperative for the safe handling of the solid cyanide.
Drawbacks are the high expenditure and that the systems are in some cases technically difficult to control (German Patent Specification No. 1120437). The production of granulates through compacting requires e.g. a specific moisture and composition. Solid sodium cyanide is compactable without any difficulty only with a water content between 0.2 and 1% by mass. In case of the moisture content being too low, the compact disintegrates. If the moisture is greater than 1% by mass, the compactor becomes congiutinated. This moisture range is however severely limited by the internationally standard quality requirements (max. 2% by mass impurities and water), because conditioned by raw material impurities and necessary residual alkalinity in the neutralization step of the NaCN process, other substances are present in far more than 1% by mass.
Extreme requirements relating to the narrowly restricted working range of the drying step (0.2-0.4% by mass H.sub.2 O) thereby ensue. Relatively large contents of sodium formate also impede the formation of compacts. In U.S. Pat. No. 3,207,574 the iron content is regarded as one of the main factors for the compactability.
In German laid open print No. 1928872 temperatures of 125.degree.-225.degree. C. are specified for the compacting. This range involves very high expenditure of energy for the purposes of compacting.
The NaCN solid products (compacts, tablets, briquettes) produced according to the known technologies furthermore have critical drawbacks in respect of their application and handling properties.
1. In most cases of application NaCN is employed in the form of solutions, i.e. the NaCN present in pressed forms is dissolved again. Due to the smooth surface, the size of the compacts and the high density, this process requires considerable dissolving time and high expenditure of energy.
2. Due to the sharp edgedness and the formation of burrs at the edges of the compacts, considerable abrasion is experienced in filling the transport receptacle as well as during transport, leading to dust nuisance and exposure to danger in handling.
Considerable defects are also to be noted for the crystallization stage. If temperatures of approx. 65.degree. C. are not observed, formate is produced, negatively affecting the quality of the end product. This requires however considerable expenditure for the vacuum generation. Vapours obtained in the process with an ammonia content of approx. 1500 ppm and a cyanide content of approx. 1000 ppm have to be detoxicated and freed of ammonia.
In addition, in using a discontinuous crystallizer, the heat of reaction obtained in the neutralization step cannot be utilized, because the intermediate storage has to take place at temperatures of about 5O.degree. C. owing to the danger of formate being produced (DD-WP 246 226).
With the principle of spray drying according to German Pat. Specification No. 969 731 these drawbacks are indeed eliminated, but danger-free, safe handling of the dried, fine NaCN powder is not constituted. Therefore it has to be compacted, giving rise to the above-described difficulties.
Further problems may arise in the intermediate storage of the NaCN powder by clotting and bridging.
Altogether, in the previous procedures there has been very high expenditure in apparatus and a considerable potential in technological and technical factors leading to faults.